Loop Not Working

[Prosdo]

Hey Everyone. I have a program for school that is suppose to project how many seconds it takes a projectile to hit the ground. I have the formula, but the loop is not posting to my output file. Can someone tell me what I'm doing wrong?

#include <iostream.h>
#include <stdlib.h>
#include <cmath>
#include <iomanip>
#include <fstream>
 
using namespace std;
float const gravity = 9.8;
double velocity;
double seconds;
double height;
ofstream launch;
 
int main()
{
launch.open("C:\\Documents and Settings\\All Users\\Desktop\\launch.dat");
launch<<setw(13)<<"Time(Seconds)"<<setw(8)<<" "<<setw(14)<<"Height(Meters)"<<'\n'<<'\n';
cout<<"Please enter the the launch velocity in meters/seconds squared."<<'\n';
cin>>velocity;
 
seconds = 1;
while(height > 0)
{
height = velocity * seconds - 1/2 * gravity * pow(seconds,2);
launch<<setw(13)<<seconds<<setw(8)<<" "<<setw(14)<<height<<'\n';
seconds++;
}
 
launch.close();
      system("PAUSE");
      return 0;
}

[Ganon11]

In your formula, you use 1/2. That's fine for real life, but not in C++ - When an integer is divided by another integer, the computer returns an integer, and if the actual answer is a decimal, it truncates the decimal part. So 1/2 is 0.5, which gets truncated down to 0, so the acceleration due to gravity is never considered, only initial velocity. Try 1.0/2.0 instead of 1/2.

[Prosdo]

Something weird is going on with the program. It compiles, but when I open the prompt and enter a velocity then hit enter it just skips to the next line and doesn't do anything? I have no clue what's going on.

[Ganon11]

Check the contents of launch.dat and see if your output is being written. If not, the file may not be opening.

[Prosdo]

Okay I fixed the cin issue kinda. Now it is only writing my headers to the file. I don't get what is going on. Before I fixed the cin issue it was writing data to the output file even though it was wrong. I was suppose to use a while loop but it only works with a for loop and when I use the for loop it put the wrong data. Ugh!

[Ganon11]

Well, your loop runs while height is greater than 0 - but you never initialize height. Perhaps you can initialize height to 0, and have the loop run while height is greater than or equal to zero?

[Prosdo]

Well, your loop runs while height is greater than 0 - but you never initialize height. Perhaps you can initialize height to 0, and have the loop run while height is greater than or equal to zero?

I did that. My professor said to use a while loop so it doesn't go past when the projectile hits the ground. When I do that it doesn't work, but as soon as I make it a for loop it runs except it goes one past when the projectile would hit the ground into the negative and I'll lose points for that.

[Ganon11]

OK, I took your code and made a few modifications:

1) Changed the 1/2 to (1.0/2.0) as we discussed.
2) Made the loop run while height >= 0
3) Set height = 0 before the loop.
4) Used const double gravity = 9.8; rather than float const gravity = 9.8

I then made the output cout << rather than launch << (which should not change the results, only where I see them), and your program works perfectly.

[Prosdo]

OK, I took your code and made a few modifications:

1) Changed the 1/2 to (1.0/2.0) as we discussed.
2) Made the loop run while height >= 0
3) Set height = 0 before the loop.
4) Used const double gravity = 9.8; rather than float const gravity = 9.8

I then made the output cout << rather than launch << (which should not change the results, only where I see them), and your program works perfectly.

When you run it does it go into the negatives in the results like:

Time(Seconds) Height(Meters)

1 20.1
2 30.4
3 30.9
4 21.6
5 2.5
6 -26.4

My assignment says it should end at 0 but no matter what way I change it, it keeps going to negatives. Thank you for the help I really appreciate it.

[Ganon11]

Yes, it does output one negative number. However, this is due to the very nature of your program. Maybe you can calculate the height for the first second, and then execute your loop; inside the loop, print first, and then re-calculate height. Or, you can change your loop to a do...while loop. This will get rid of the last, negative result.

However, you then have to manually calculate the time when height = 0 again. This will likely be an explicit formula, from the fact that 0 = vel * t - 9.8 * t^2. You can use the quadratic formula with a = -9.8, b = velocity, c = 0.